The present invention relates to a differential amplifier circuit and a mixer circuit using the same and capable of integration and, more particularly, to a differential amplifier circuit and a mixer circuit which can be used in equipment for mobile communication.
In the field of information communication, the development of technologies using higher RF frequencies including wideband CDMA using RF frequencies in the 2 GHz range, Bluetooth using RF frequencies in the 2.4 GHz range, and a wireless LAN using further higher RF frequencies has advanced recently.
The amplification of extremely weak signals with transmitted/received RF frequencies in equipment such as a terminal device or a wireless base station used for such information communication is not only costly but also requires multi-stage amplifiers. It is therefore normal practice to convert RF frequencies received from, e.g., an antenna to signals at lower frequencies. In such a case, an RF mixer circuit using a differential amplifier circuit is used to convert RF signals to signals with lower frequencies.
FIG. 6 is a circuit diagram showing a structure of a conventional double-balanced RF mixer circuit.
The RF mixer circuit shown in the drawing is composed of: a differential amplifier circuit 111; and a double-balanced mixer circuit (hereinafter referred to as the DBM circuit) 110 connected to the differential amplifier circuit 111. When a radio-frequency signal (RF signal) is inputted to the differential amplifier circuit 111 and a local oscillation signal (local signal: LO signal) is inputted to the DBM circuit 110, these signals are mixed in the DBM circuit 110 so that an intermediate frequency signal (IF signal) with a lower frequency is outputted.
The differential amplifier circuit 111 has: a first bipolar transistor 105; an RF signal input port connected to the base of the first bipolar transistor 105; a second bipolar transistor 106 having a base connected to the ground via a capacitor 401 and an emitter connected to the emitter of the first bipolar transistor 105; resistors 202 and 203 connected between the respective emitters of the first and second bipolar transistors 105 and 106; and a current source connected between the resistors 202 and 203 and having a resistor 201. In the differential amplifier circuit 111, if an RF signal with a frequency f1 is inputted to the RF signal input port, signals each having a frequency F1 and having a 180° phase difference therebetween are outputted from the respective collectors of the first and second bipolar transistors 105 and 106.
On the other hand, the DBM circuit has: third and fourth bipolar transistors 101 and 102 having respective emitters connected to each other; fifth and sixth bipolar transistors 103 and 104 having respective emitters connected to each other; first and second LO signal input ports; and first and second IF signal output ports. The respective bases of the third and sixth bipolar transistors 101 and 104 are connected to each other and also connected to the first LO signal input port. The respective bases of the fourth and fifth bipolar transistors 102 and 103 are connected to each other and also to the second LO signal input port.
To the first and second LO signal input ports, signals each having a frequency f2 and having a 180° phase difference therebetween are inputted. The respective collectors of the third and fifth bipolar transistors 101 and 103 are connected to the first IF signal output port. The respective collectors of the fourth and sixth bipolar transistors 102 and 104 are connected to the second IF signal output port. When LO signals are inputted to the first and second LO signal input ports and output signals from the differential amplifier circuit 111 are inputted to the third, fourth, fifth, and sixth bipolar transistors 101, 102, 103, and 104, output signals having equal frequencies and equal amplitudes and a 180° phase difference therebetween are outputted from the first and second IF signal output ports.
In the present specification, one and the other of the signals having equal frequencies and equal amplitudes and a 180° phase difference therebetween will be hereinafter referred to as the “non-inverted” signal and the “inverted” signal, respectively.
The conventional mixer circuit has used the differential amplifier circuit which generates two balanced output signals which are non-inverted and inverted to improve a distortion property. In other words, the equal amplitudes of the two balanced output signals from the differential converting amplifier circuit and the 180° phase difference therebetween have reduced the non-linearity between the output signals. As a consequence, the conventional RF mixer circuit has been reduced in noise and harmonics compared with a single-balanced RF mixer circuit. The harmonics are defined herein as signals having frequencies which are integral multiples of an inputted signal.
On the other hand, Japanese Laid-Open Patent Publication No. HEI 5-175755 discloses an example of a double-balanced RF mixer circuit using field effect transistors in place of bipolar transistors. Even when the field effect transistors are used, distortion in output signals can be reduced in the same manner as in the mixer circuit mentioned above.
The differential amplifier circuit 111 is not only used in the foregoing mixer circuit but also used alone in various circuits to amplify a signal.
FIG. 7 shows a structure of a conventional differential amplifier circuit. The differential amplifier circuit shown in the drawing has the same structure as the differential amplifier circuit 11 in the RF mixer circuit shown in FIG. 6, except that the first bipolar transistor 105 has the collector connected to a first output port 150 and the second bipolar transistor 106 has the collector connected to a second output port 151. Such a conventional differential amplifier circuit is used to amplify a signal for a circuit having a double-balanced configuration, such as the mixer circuit mentioned above.
As described above, an RF frequency inputted to an RF mixer circuit causes therein harmonic signals. Of the harmonic signals, even-order harmonics such as the second and fourth harmonics are cancelled by using a conventional double-balanced RF mixer with excellent symmetry so that even-order distortion is reduced.
On the other hand, odd-order harmonics such as the third and fifth harmonics are not cancelled by the conventional RF mixer circuit and appear in outputs so that it has been impossible to suppress distortion in the outputs caused by the odd-order harmonics. In the odd-order distortion, the third distortion has the largest amplitude, which is the main cause of the distortion. In terms of transistor characteristics, distortion is more likely to occur with higher frequencies so that trouble caused by harmonics are particularly conspicuous with RF frequencies.
Thus, it has been difficult to implement smaller-size and higher-performance communication equipment with the structure of the conventional RF mixer circuit.
The odd-order distortion is caused by the non-linearity between the transistors of the differential amplifier to which an RF signal is inputted. In the differential amplifier circuit 111 to which the RF signal is inputted, therefore, a method of giving a feedback as current outputs by inserting the resistors 202 and 203 in the emitter ends of the first and second bipolar transistors 105 and 106 and thereby reducing the non-linearity.
As a result, the amount of the feedback of the third distortion is increased in proportion to the currents so that the third distortion is reduced.
However, power is consumed in the resistors 202 and 203 so that the effect of reducing signal distortion is limited in design aiming at lower power consumption. Therefore, it has been difficult to simultaneously achieve lower distortion and lower power consumption in the conventional RF mixer circuit.
Since odd-order distortion leads to troubles in equipment when the differential amplifier circuit is used alone, it has been requested to reduce harmonics without increasing power consumption.